Traditionally breast cancer has been thought of as an illness subject to reverse disparities based on socioeconomic status (SES); that is, incidence and mortality rates were lower for women of lower socioeconomic levels. However, during the past two decades this situation changed. Despite breast cancer incidence rates which remained 10-20% lower for poorer populations compared to more affluent populations, breast cancer patients residing in poorer neighborhoods had 20-30% lower survival than those residing in wealthier neighborhoods by the late 1990s. It is likely that the worse survival experience of lower SES populations is attributable at least in part to lesser use of the effective but expensive oral medications that have become available for treating postmenopausal breast cancer. The advent of Part D (pharmaceutical) coverage for Medicare beneficiaries in 2006 provides an opportunity to examine the hypothesis that the availability of pharmaceutical insurance coverage will reduce socioeconomic disparities with respect to the use of adjuvant hormonal and other oral medication therapies, and therefore will reduce socioeconomic disparities in survival. The specific aims of this project are: 1) Determine, among a cohort of Medicare-eligible breast cancer patients, the characteristics of those enrolling in the Medicare Part D program, with special attention to neighborhood socioeconomic status and comorbid conditions. 2) Determine the relationship of SES, Part D enrollment, comorbidity, and mortality among a cohort of older breast cancer survivors. 3) Examine the use of adjuvant hormone therapy among breast cancer Part D enrollees and evaluate its relationship to mortality disparities by SES. These aims will be carried out by analysis of serial cohorts of Medicare breast cancer patients. A validated and published algorithm will be employed to identify national samples of Medicare breast cancer survivors with initial treatment in 2001-2008. Geocoding of addresses to census tracts will permit determination of neighborhood SES. Three-year and five-year outcomes of overall mortality and disease-specific mortality will be ascertained using Medicare claims and National Death Index data. Analyses will employ different approaches (instrumental variables and propensity scores) to controlling for endogeneity in the choice to enroll in the Medicare Part D program. Analyses to address Specific Aim 3 will utilize Medicare Part D pharmaceutical data, a novel data source. This project will have major significance for the field of breast cancer survivorship, and for those interested in reducing disparities in outcomes for that disease. It will also provide critical information to policy-makers regarding the fledgling Part D program.